A myriad of applications exist around the world for pumping suspended solids. In some applications, it is necessary to have the pump cut these solids into smaller pieces as they pass through the pump. Some installations include a separate cutting device ahead of the pump, but in other instances, the cutting occurs within the pump. Other applications require cutting the solids to prevent clogging the pump or another piece of downstream equipment.
One type of clogging problem is related to the pumping of stringy matter. Stringy matter can wrap itself around the leading edge of an impeller vane. Some factors associated with wrapping are related to the matter itself, such as the length of the stringy matter and the concentration of matter in the liquid stream. Other factors controlling wrapping are machine related such as the flow rate or impeller rotational speed. The wrapping of stringy matter is of concern, as once stringy matter is wrapped around the impeller vane, other solids tend to get caught in the matter, resulting in a mass that increases in size. This results in a decrease in pump performance and accordingly, lower flow rate and pump efficiency. A common remedy is to shut the pump down and manually remove this clump in order to return the pump to its desired performance.
Historically, most solids-handling pumps have been designed to allow relatively large solids to pass through the entire pump. This leads to some compromise on performance as the impeller vane profiles are no longer optimized for the best hydraulic performance. A similar approach has been to design the pump with only one vane on the impeller. This, however, can lead to great expense and difficulty in trying to trim and balance the impeller for proper operation. Vortex impeller designs, where the impeller sits recessed out of the flow path, are also commonly used to pump suspended solids.
A removable cover plate, or back cover, allows for more efficient removal of debris. Further advances have included the use of a protrusion into the eye of the impeller to “wipe” any stringy matter off of the impeller vane's leading edge. Other recent developments have focused on two areas: modifying impellers to have one continuous vane instead of multiple vanes or a single vane, and making modifications to wear plates by adding notches and grooves to help disrupt and break up any accumulation of stringy matter that may get caught on the impeller vane's leading edge.
Chopper pumps, referred to as such due to the fact that they cut the solids as they pass through the pump, have been in existence for many years. They are found in numerous applications including, but not limited to: sewage, seafood processing, meat processing plants, paper mills, and manure/agricultural. Some applications for chopper pumps fall under what is referred to as the Ten States Standard that requires municipal wastewater pumps to either pass a 3″ spherical solid or to be a chopper pump.
Some applications today that are handled by traditional non-clog pumps are done so in combination with filters, screens, or some other cutting device upstream of the pump. A well applied chopper pump could eliminate the need for these additional devices in certain applications, simplifying the customer's installation in the field. This reduction in complexity often will also lead to improved overall efficiency. Screens and filters can clog over time and become less efficient. In some cases, this reduces the net positive suction head available (NPSHA) to the point where the pump cavitates and operates outside of its preferred operating range. The additional cutting devices can also lead to reduced NPSHA and also require additional power when in operation.
One of the primary limitations of today's chopper pumps is that they rely upon the pump impeller to perform the shearing action. This makes repair and renewal of clearances both expensive and time-consuming. This design feature also renders the pump inoperable without the cutter in place unless a spare impeller is readily available.
Also typical of most chopper pumps commercially available today is that there is a stationary cutter that is positioned perpendicularly to the pump shaft that the solids come in contact with first. This means that the cutting action occurs as the solids flow axially in toward the impeller. For example, U.S. Pat. No. 7,455,251 discloses a chopper pump structured with a chopper plate and impeller that are configured with an open eye or “hubless” arrangement. However, with this configuration, large axial loading can result.
Other limitations of many of today's chopper pumps include the need to remove a heavy cleanout cover plate to inspect the cutters or to remove any clogs and the need to remove the pump from its plumbing to replace worn cutters. Moreover, few conventional chopper pumps operate well on a suction lift.